1. Field of the Invention
The present invention relates generally to calibration systems and, in particular, to the automatic calibration of a projector-camera system.
2. Description of the Prior Art
Many computer vision applications involve transformations between an observed point in the scene and the corresponding point in the camera image. The parameters for this projective transformation are typically derived by establishing correspondences between a small set of points on a known target and their respective images. In a camera-projection system, pixels in the computer display frame are projected onto a flat surface and then observed by a camera. This involves a composition of two transforms: one from the projector to the screen and a second from the screen to the camera.
A known calibration pattern is projected onto a possibly unknown flat surface by a projector with a possibly unknown location, orientation and focal length. An image of this pattern is captured by a camera mounted at a possibly unknown location and orientation and with a possibly unknown focal length. It is necessary then to recover the mapping between a given point in the source (pre-projection) image and its corresponding point in the camera image.
Recovering the parameters of the mapping in the prior art systems requires knowledge of the projector and camera setup. Typically, passive scenes are tradditionally studied in computer vision applications for a projector-camera system. In addition, complete physical models must be derived to create a composition of non-linear distortions.
It is, therefore, an object of the present invention to allow the recovery of the parameters of the mapping without knowledge of the projector and camera setup. It is another object of the present invention to allow the projector-camera system to project known calibration patterns into the scene, unlike the passive scenes traditionally studied in computer vision. It is a further object of the present invention to be modeled as a single projective transform, not requiring the derivation of a complete physical model. It is well known that a projective transform can be completely specified by eight parameters, so it is still a further object of the present invention to automatically recover these parameters.
In order to overcome the problems with prior art systems, we have developed a method which includes arbitrarily placing a camera and projector in the scene, such that their fields of view intersect a (planar) region on the screen; projecting one or more known calibration patterns on the screen; capturing the images of the calibration pattern(s) by a digital camera; identifying the locations of the features in the calibration pattern(s) in the camera image(s); and, given the locations of a small set of corresponding features in both source and camera frames, utilizing the techniques of linear algebra to obtain the parameters for the mapping. At least four feature points (in one or more patterns) must be visible in the camera image. If more features are available, then the mapping that best fits the data (in a least-squares sense) is found.
We have also developed an apparatus that is capable of performing the above-described method.